1. Field of the Invention
The present invention relates to a method for manufacturing a heat transfer module with a hydrogen adsorption alloy mainly composed of metal hydride.
2. Prior Art
Hitherto, in the prior art there has been developed in which hydrogen is adsorbed certain metals or alloys to be stored therein and transferred therefrom in the form of a metal hydride. This prior art has been further applied to such practical use as purification of hydrogen, pressure rise, heat pumps, air-conditioning systems, etc.
In such a practical application, because an exothermic reaction or an endothermic reaction takes place necessarily at the time when the metal hydride adsorbes or discharges hydrogen, such a property can be utilized in a heat exchanger, heat pump, etc.
When it is desirable to store or transfer hydrogen, the delivery of hydrogen is not effectively performed without a rapid exchange of heat between the metal hydride and the outside, from the viewpoint of high thermal efficiency, and efficient storage and transfer of hydrogen.
However, the thermal conductivity of the hydrogen adsorption alloy itself, in the form of particles a is not high, and therefore several attempts have been proposed aiming at efficient delivery or exchange of heat.
According to one of such proposed attempt, for the purpose of improving the hydrogen adsorption alloy itself, surfaces of the particles are plated with a different kind of metal of high thermal conductivity.
According to another attempt, the structure of the heat exchanging unit is improved so that a hydrogen adsorption alloy in the form of particles is brought as close as possible into contact with a heat transfer element. For that purpose, inner and outer peripheries of a heat transfer pipe are provided with fins, for example.
According to a still further attempt, compression molding is employed.
FIG. 6 shows an embodiment of example U.S. Pat. No. 4,609,038 of which patentee is the applicant, and in which a heat exchanging unit is manufactured by the steps of coating fine particle surfaces of hydrogen adsorption alloy with a different kind of metal by plating, compressively molding them to obtain a hydrogen adsorption alloy compact 7a, providing through holes through the compact, inserting a heat exchanging pipe 8a in the through holes to be in direct contact with the holes, thereby ends of the pipe may communicate with a supply port and exhaust port for heating or cooling medium respectively.
However, there still remain several problems to be solved in the foregoing prior arts respectively.
In the attempt of improving hydrogen adsorption alloy itself to enhance its thermal conductivity, there is a restriction in terms of distance within which heat can be transferred from a heat transfer surface, because thermal conductivity of the hydrogen adsorption alloy is essentially low when it is in the form of particles. By the same reason, satisfactory improvement in thermal conductivity is not achieved, either, even if a large number of fins are densely installed for rapid delivery of heat.
In the attempt of compressively molding the particles into a compact, it is certain that thermal conductivity of the hydrogen adsorption alloy in the form of a compact is significantly improved as compared with the alloy in the form of particles or powder, but a further problem still exists in how to make close contact between the heat transfer element and the hydrogen alloy compact. For example, in the arrangement of a heat exchanging unit forming a compact of alloy by compression molding as illustrated in FIG. 6 and inserting several heat transfer pipes (of copper) through the compact, it is necessary to provide through holes for such insertion of the heat transfer pipe, and therefore whether or not a close contact is attained between the external surface of the pipe and the alloy compact is a key in this arrangement.
In the heat transfer module in which the external surface of a heat transfer pipe is exposed to a heating medium and a cylindrical hydrogen adsorption alloy compact is inserted inside the pipe to be in contact with the internal surface of the pipe, there is the same problem as above.
As is well known, the hydrogen adsorption alloy essentially expands when adsorbing hydrogen gas and producing metal hydride, and contracts when exhausting hydrogen gas. Particles of hydrogen alloy are formed into a compact for the purpose of improving thermal conductivity and preventing the particles from micronization and scattering due to repeated contraction and expansion thereof in the use. With such formation into a compact, relation between the compact and heat transfer pipe becomes delicate and complicated. More specifically, when establishing the true specific gravity of the hydrogen adsorption alloy as 1, then the bulk specific gravity of the alloy in the form of particles is in the range of 0.37 to 0.39 which is naturally smaller than the true specific gravity. The specific gravity increases when formed into a compact by compression molding. For example, the specific gravity of the alloy compact is in the range of 0.64 to 0.65 when formed under pressure of 2 Tons/cm.sup.2 applied by cold isostatic press (hereinafter referred to as "CIP").
In the reaction between the alloy compact and hydrogen gas without any restriction, the expansion of volume mounts to 141% when the alloy is made of LaNi.sub.4.95 Al.sub.0.5 Cu, and 123% when the compact is made of MmNi.sub.4.3 Al.sub.0.7 Cu. The expansion coefficient is largely different depending upon the kind of alloy as mentioned above. Even in alloys of the same kind, their expansion coefficients may be largely different from each other just by reason of a little difference in the content of Al, thus the conditions to be established between the compact and the heat transfer pipe are very delicate and intricate. That is to say, if a hydrogen adsorption alloy compact to be inserted in a heat transfer pipe is permitted to expand freely in the pipe, the strength of the compact is lowered bringing about collapse, micronization and scattering thereof, eventually impairing the advantage of having been formed into a compact. On the other hand, if there remains some gap between the heat transfer pipe wall and the inserted compact after the expansion, an undesirable heat insulating layer (space) is formed therebetween resulting in a serious decline of thermal efficiency.
On the contrary, if expansion of the alloy compact is excessively restricted, the heat transfer pipe is deformed or the compact is destroyed due to the physical stress resulting from such restriction further resulting in collapse of the compact.